Caution Cisco Inter-Switch Link (ISL) and Cisco Dynamic Trunking Protocol (DTP) are not supported by the ML-Series cards, but the ML-Series broadcast forwards these formats. Using ISL or DTP on connecting devices is not recommended. Some Cisco devices attempt to use ISL or DTP by default.

Understanding Bridging

The ML-Series card supports transparent bridging for Fast Ethernet, Gigabit Ethernet and POS ports. It supports a maximum of 255 active bridge groups. Transparent bridging combines the speed and protocol transparency of a spanning-tree bridge, along with the functionality, reliability, and security of a router.

To configure bridging, you must perform the following tasks in the modes indicated:

•In global configuration mode:

–Enable bridging of IP packets.

–Select the type of Spanning Tree Protocol (STP) (optional).

•In interface configuration mode:

–Determine which interfaces belong to the same bridge group.

The ML-Series card bridges all nonrouted traffic among the network interfaces comprising the bridge group. If spanning tree is enabled, the interfaces became part of the same spanning tree. Interfaces not participating in a bridge group cannot forward bridged traffic.

If the destination address of the packet is known in the bridge table, the packet is forwarded on a single interface in the bridge group. If the packet's destination is unknown in the bridge table, the packet is flooded on all forwarding interfaces in the bridge group. The bridge places source addresses in the bridge table as it learns them during the process of bridging.

Spanning tree is not mandatory for an ML-Series card bridge group. But if it is configured, a separate spanning-tree process runs for each configured bridge group. A bridge group establishes a spanning tree based on the bridge protocol data units (BPDUs) it receives on only its member interfaces. The ML-Series card has a 255 active bridge group maximum.

Configuring Bridging

Use the following steps to configure bridging:

Command

Purpose

Step 1

Router(config)# no ip routing

Enables bridging of IP packets. This command needs to be executed once per card, not once per bridge-group. This step is not done for integrated routing and bridging (IRB).